The invention relates to a method and apparatus for continuously treating a strip of hot dip galvanized steel.
A known treatment, "galvannealing", consists of heating a strip of hot dip galvanized steel up to a chosen temperature, maintaining it at this temperature, and then cooling. This treatment is intended to provide diffusion of the iron of the strip into the zinc of the coating, to the extent of yielding an iron content in the amount of 7 to 13% of the coating. Iron contents outside this range produce alloys which, if the iron concentration is too high, involve powdering when the strip is subjected to deep drawing or the like, or if the iron concentration is too low, the steel strip cannot be satisfactorily welded.
In current practice, galvannealing is carried out in an apparatus wherein the strip undergoing treatment passes through at least two vertical runs (ascending and descending). This type of apparatus typically can also be used for producing a conventional hot dip galvanized strip.
In the classical galvannealing operation, a furnace for heating and maintaining the strip at temperature is disposed above the galvanizing bath, immediately downstream of the air jet wiping apparatus. This furnace is generally retractable, not being used in the production of conventional hot dip galvanized strips. A first cooling apparatus is disposed above the furnace, typically comprised of a group of air blowers. The purpose of the cooling is to avoid damage to the coating by the redirecting rolls which guide the strip for the return run downward from the altitude reached in the process. The height of the combination of the furnace and the cooling apparatus determines the height of the upward run of the strip, which generally does not exceed 50 m because of susceptibility to vibrations produced at the level of the aforementioned wiping air jets. Customarily, a second cooling apparatus, e.g. a second group of blowers, is disposed at the beginning of the downward movement which follows the galvannealing operation.
In such an installation, the coated strip leaves the zinc bath at a temperature of approximately 450.degree.-480.degree. C. and passes through a drying segment employing air jets, following which it is subjected to the galvannealing operation in which it is heated to a temperature in the range 460.degree.-600.degree. C. chosen depending on the grade of the steel employed. It then undergoes cooling, first by an initial group of blowers at the end of the upward run and then by a second group at the beginning of the downward run. The temperature of the strip following this cooling is one which is suitable in light of intended further treatment or handling of the coated strip.
Under current practice, heating of the strip is provided by a direct fired furnace or an induction furnace, capable of raising the strip temperature by 50.degree.-100.degree. C. but at a relatively low heating rate, e.g. 6.degree. C./sec for a strip 0.7 mm thick, in the case of a burner-equipped furnace, and 30.degree. C./sec in the case of an induction furnace operating at a 10 kHz frequency. The energy efficiency of gas burners is low, on the order of 30%. Further, a classical induction furnace with multi-turn windings and with longitudinal or transverse magnetic flux, has sometimes to be adapted to correct transverse irregularity in the temperature.
The means of maintaining the strip at the elevated galvannealing temperature generally comprise an insulated channel, possibly equipped with heating means, e.g. electrical or gas heating means. The temperature-maintenance section occupies approximately one-fourth of the height of the upward vertical trajectory of the strip. The combination of the heating furnace and the temperature-maintenance means must be sufficiently long to provide a duration of passage greater than 10 seconds, preferably greater than 15 seconds, at a temperature above 450.degree. C.
Such an arrangement does not allow optimal conditions for galvannealing. The fact that the heating rate of the galvanized strip is only moderate makes it necessary to have a somewhat long heating section. This limits the length of the constant-temperature-maintenance section where iron is caused to diffuse from the strip into the zinc. As a result, higher temperatures must be used to achieve the same desired diffusion. Also, it is well known that higher temperatures in the temperature-maintenance section, while achieving a shorter time at the galvannealing temperature, do so at the cost of increased risk of powdering during deep drawing operations.